Make when-macros work by accepting varargs

flow/actorcompiler.h

@@ -31,7 +31,7 @@ template<typename T> struct FutureStream;
 #define state
 #define UNCANCELLABLE
 #define choose if(1)
-#define when(x) for(x;;)
+#define when(...) for(__VA_ARGS__;;)
 template <class T> T wait( const Future<T>& );
 void wait(const Never&);
 template <class T> T waitNext( const FutureStream<T>& );